Re: R: R: Quadrifilar home brew

At 12:18 PM 3/17/2005 +0100, i8cvs wrote:
> ----- Original Message -----
> From: Edward R. Cole
> To: i8cvs ; AMSAT-BB
> Sent: Wednesday, March 16, 2005 4:35 PM
> Subject: Re: R: [amsat-bb] Quadrifilar home brew
>
>
> >The main reason that i have experimented on 2400 MHz Quadrifilar Helix
> >Antennas as per AMSAT Journal May/June 2004 was to find the way to
> >produce a LHCP QFA to be used as a feed in a small dish having a low
> >F/D ratio in the order of 0.3 to 0.35
> >
> >Pulling with you for P3E to verify the results !
> >
> >73" de
> >
> >i8CVS Domenico
>
> Domenico,
>
> I have just been studying the septum feed for eme use which is very simple
>to build and provides both LHCP and RHCP. I am giving some thought to
>building one for my 85cm offset dish on 2.4 GHz. Have you looked at this
>feed? W1GHZ did an exhaustive analysis of it in 2002 or 2003. I will likey
>make a plain one first, then add a sliding circular choke ring for
comparison.
>The basic feed is good for f/d 0.35 dishes. Either a flared horn or choke is
>needed for f/d = 0.6 to 0.7. At present I am using a 5-1/4 turn helix feed.
>
> Here is a link to the septum design: http://www.ok1dfc.com/EME/emeweb.htm
>
> 73's Ed - AL7EB
> http://www.qsl.net/al7eb
>
>
> Hi Ed, AL7EB
>
> I have carefully studied the septum feed designed by Zdenek OK1DFC that was
>also published in DUBUS magazine 1/2003 by Paul Wade W1GHZ and it seams to be
>interesting for EME but many more measurements are needed to state if it is
>best or not in comparison to the W2IMU or VE4MA feeds for EME
>
> My experimentation on Quadrifilar Helix Antennas for 2400 MHz was made to
>determine how to wound the loops
> and how to connect them to get LHCP to be used as a feed for a dish in
order
>to radiate RHCP from it as was requested for AO40 ( and P3E in the near
>future).
>
> Since for satellite use a RHCP and LHCP in band feed is not requested as
for
>EME than a LHCP QFA antenna is very attractive to illuminate as a feed a
small
>diameter prime focus dish having F/D in the order of 0.3 to 0.35 because the
>QFA do not requires a reflector so that the feed blockage of a QFA for 2400
>MHz over a small dish do not affect very much the overall illumination
>efficiency.
>
> Best 73" de
>
> i8CVS Domenico
Domenico,
It was not my intention to say that the QFA is not a good feed, only that
the septum has not been considered by the satellite community to my
knowledge.
The septum has a (fairly) small footprint when used for a center-fed feed.
For f/d > 0.4 then either a flared horn or choke ring is needed to narrow
the beamwidth and that will shaddow a center-fed dish (the same as a
choke-ring circular horn). I suspect that with an offset fed dish this may
not be nearly as big an issue.
My intention was to bring this new feed to the attention of the satellite
community (esp. thos who like to experiment with various feed types).
Certainly, if only one sense of circular polarity is desire the septum is
under utilized. On the other hand if one desires to switch polarity
(perhaps in high squint-angle situations) then this feed is quite
convenient because there is no added phasing harness (or circuit) loss
before the preamp (thus providing the best low-noise performance). This
last feature is what the eme folks are finding particulary attractive. It
is a new concept so I offer it in that manner.
I suspect that a septum is much easier to build than a QFA. That is my
other point.
73's Ed
----
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